Filament testing for peripheral nerve sensory function has been used more frequently and with renewed interest as the need for screening and monitoring of peripheral nerve sensory function has increased with respect to body therapy. In recent years, it has been shown that filaments are a sensitive monitor means for the testing of the peripheral nerve function of a patient, particularly hand and feet body areas. In the 1800's, the focus of peripheral nerve testing of the hands was on the study of normal physiology and horse hairs were used as filaments to measure only light thresholds of touch recognition.
In the late 1950's, it was determined that a broader range of filament forces were needed than those available with horse hairs to refine the filament method for peripheral nerve testing. Thus, J. Semmes and S. Weinstein designed and developed nylon monofilaments of increasing diameter for peripheral nerve testing with such filaments set at right angles proximate the end of acrylic (Lucite) rods. Such rods, or filament handles, were designed to fit conveniently within a therapist's hand, being approximately the length of a pencil. As noted by reference to the sole figure in the present application as more thoroughly described below, the filament would extend from the rod-like handle at a right angle for perpendicular application to the extremity of a patient. The filament is pressed against the skin of the patient until it bows. Generally, the device is maintained against the extremity of the patient for a specific period of time, for example, for 1.5 seconds, and then removed for 1.5 seconds and then repeated. At that time, the patient is asked whether any sensory perception resulted from the application of the device. If not, the filament would be interchanged by one of a larger diameter so that increased pressure would be required prior to causing the filament to once again bow.
The Semmes-Weinstein monofilament testing devices have become the standard means for repeatable testing and measurement of the threshold of cutaneous sensory perception. Through the 20-unit series of test devices, a trained hand therapist or health care examiner can distinguish in a patient between light touch, diminished light touch, diminished protective sensation, loss of protective sensation and deep pressure sensation. A series of nylon monofilaments (of uniformed 38 mm length) are sized and numbered to correspond to Log (10.times.force in mg) of force. The lowest force in the series is 4 mg and the highest force is 447 grams. The monofilaments (of constant length but of increasing diameters) are designed to bend when a specific value of force is reached and such design provides unique control of, and credibility to, the Semmes-Weinstein sensory test method. Thus, the series of Semmes-Weinstein test devices provides an accurate method by which diminishing and returning sensation of a patient's body surfaces and extremities can be evaluated and allows the health care examiner to predict and interpret the patient's levels of nerve function and sensibility.
Virtually all Semmes-Weinstein monofilament test devices employ nylon as the force applying element. These are usually provided as a set of five units having selected sensory level designations of 2.83, 3.61, 4.31, 4.56 and 6.65. It has been observed, however, that nylon monofilament suffers from certain drawbacks which reduces the overall efficacy of the cutaneous sensory perception measuring device. Simply put, it is critical in using the Semmes-Weinstein monofilament testing instrument to be constant and predictable in the amount of force applied to the specific body surface area of the patient as it relates to the sensory level designation of the monofilament segment being employed. It must be remembered that the sensory perception of the patient is measured by applying sufficient force to the preselected body area so that the monofilament segment bows by the application of sufficient orthogonal pressure. The sensory level designations relate to contrasting monofilament diameters so that bowing occurs at certain preselected pressure levels.
Unfortunately, notwithstanding its universal acceptance, nylon proves to be a less than ideal choice for use as a testing instrument. For example, as noted above, the force delivered by a monofilament segment is the function of the diameter of the filament and its length. When force is applied to the filament, it is distributed along its entire length. Bending occurs when the monofilament column begins to elongate on the side of the filament and compresses on the opposite side. At the point of initial bending the maximum moment of force is delivered. However, when additional force is applied, it is not uncommon to cause the filament to "kink." When this happens, the bending force cannot build along the entire length of the filament but, instead, the force "collects" at the kink. The bend that results focusses at the kink such that the force to elongate one side of the kink and compress the opposite side is reduced. Not surprisingly, the dynamics of the bending moment of the monofilament are dramatically changed noting that efforts to straighten the kink will not resolve the loss in force to bending. As such, once a kink has occurred, the standard predictable moment of force required to form a uniform bend in order to make a peripheral nerve function determination is lost. Nylon is, unfortunately, quite susceptible to kinking.
In addition to the above, nylon, due to the very nature of its molecular structure, lacks the necessary consistent force to cause bending from monofilament to monofilament. As such, nylon monofilament allows a therapist to make gross measurements relating to cutaneous sensory perception of a patient's body areas but finely tuning such measurements is not practical. There are several reasons why nylon has proven to be less than ideal as a measuring device. It has been noted that nylon is sensitive to climactic conditions such as ambient temperature and humidity. Fairly dramatic differences in bending force are measured when such climactic conditions are altered. Further, nylon requires a relaxation period in order to operate properly. It has been observed that the bending force of a nylon monofilament remains constant after use until the monofilament has been employed for five to ten consecutive applications, at which time, the monofilament requires up to six hours to recover its original flexural modulus.
It is thus an object of the present invention to provide a Semmes-Weinstein type of cutaneous sensory perception device wherein the nylon monofilament has been replaced by a material which enables the device to work in a more predictable and thus acceptable manner.
This and further objects will be more readily appreciated when considering the following drawings and appended claims.